1. Field of the Invention
Embodiments of the invention relate to the field of optical communication devices. More particularly, the present invention relates to a transmitter used for optical communication systems implemented on a photonic integrated circuit.
2. Discussion of Related Art
Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and an optical combiner to combine each of the modulated outputs. Conventionally, WDM systems have been constructed from discrete components. For example, the lasers, modulators and combiners have been packaged separately and mounted on a printed circuit board, More recently, however, many WDM components including transmitter, receivers and passive devices have been integrated onto a single chip also referred to as a photonic integrated circuit (PIC).
In order to further increase data transmission rates associated with WDM systems, various modulation formats have been proposed for generating the modulated output. In the common RZ modulation format, one of the two logical states corresponds to the presence of an optical pulse with proper optical power and duration shorter than the bit period to represent, for example, a logical 1 bit. The second of the two logical states corresponds to the absence of pulses, or to a pulse with reduced power to represent, for example a logical 0 bit. Another modulation format is the binary NRZ format which consists of transmitting a first logical state (for example the 1 bit) by a substantially constant optical signal associated with the entire time bit slot. The second logical state (for example the 0 bit) is transmitted with a reduced or absent optical power associated with the particular bit slot. Another modulation format, known as polarization multiplexed differential quadrature phase-shift keying (“Pol Mux DQPSK”), can provide higher data rates as compared to these other transmission formats. However, a transmitter outputting Pol Mux DQPSK signals typically requires more components and greater complexity than an RZ or NRZ transmitter. Thus, there is a need to integrate components associated with a Pol Mux DQPSK transmitter on a PIC in order to realize a WDM system transmitter having fewer discrete components with reduced costs.